For the dough, place all ingredients into a large bowl holding back a quarter of the water.
Stir with your hands until a dough is formed.
Slowly add the remaining water and knead the dough in the bowl for four minutes. (Alternatively, you could mix with a stand mixer fitted with a dough hook).
Tip dough onto a lightly floured surface and knead well for 10 minutes, until the dough is smooth and elastic.
Place the dough into a clean bowl, cover with a damp tea towel and leave to rise for an hour.
Tip the dough out onto a very lightly floured surface, and knock it back by kneading it a few times.
Divide the dough into 10 equal portions and shape each portion into a ball.
Place all balls onto a floured baking tray and allow to rise for an hour.
Preheat a deep fat fryer, filled with sunflower oil, to 180C/350F.
Lower each doughnut into the fryer, cooking each side for about five minutes or until golden brown. Remove the doughnuts from the oil with a slotted spoon and immediately roll in caster sugar so that the hot fat makes the sugar stick to the sides.
Set aside and leave to cool.
When completely cool, using a small, sharp, knife make a cut into the side of the doughnut, reaching to the centre.
Spoon some smooth or sieved strawberry jam into a piping bag, or piping syringe.